The invention relates to trenchless drilling, in particular to an improved apparatus and method for bursting an existing pipe and replacing it with a new pipe.
Trenchless installation of underground conduits such as water pipes, telephone and electrical cables and similar utilities under roadways and similar obstructions has been made possible through the development of horizontal boring machines. In many applications, the same horizontal drilling machine is used to replace an existing underground conduit or pipe when the pipe is deteriorated or when it is necessary to replace the pipe with a larger conduit to provide more capacity. In these cases, the machine is used to pull a reaming tool through the bore to burst or break up the existing pipe and simultaneously enlarge the bore. A new pipe or conduit is drawn along behind the reamer.
A number of back reaming tools have been developed for various purposes. Back reamers for use in enlarging existing holes often have distinct cutting surfaces and/or cutting teeth. See, e.g., U.S. Pat. No. 5,390,750, issued Feb. 21, 1995 to Deken et al. and U.S. Pat. No. 5,687,805, issued Nov. 18, 1997 to Perry. Back reamers have also been used in pipe bursting and replacement operations. A back reamer is rotated and pulled through the existing pipeline by a drill string, and the replacement pipe is drawn along behind by a swivel connection, as described in Brewis U.S. Pat. No. 5,607,257, Mar. 4, 1997. The present invention provides a pipe burster especially adapted for rupturing an existing pipeline, expanding the hole beyond the diameter of the existing pipeline, and pulling into place a replacement pipe of larger diameter than the existing pipeline.
A pipe bursting system according to the invention operates by compaction rather than by reaming, that is, bursting the existing pipeline from within, rather than cutting it away. The system includes a conventional directional boring machine, a drill string, and a pipe burster attached to the terminal end of the drill string. A pipe burster according to the invention includes a frame having a front connecting portion configured for connection to a drill string that can pull and rotate the frame about an axis of rotation, at least two axles mounted on the frame, and rollers mounted on each axle. Each roller has a curved outer surface forwardly inclined towards the axis of rotation of the frame, whereby rotation of the frame about its axis of rotation results in rotation of the rollers when the curved outer surfaces of the rollers engage a cylindrical inner surface of a pipe being burst.
In a preferred embodiment, the frame includes a shell having at least two spaced lengthwise external openings therein, the axles are mounted near each opening, and a portion of the curved outer surface of each roller protrudes from its associated opening. Each opening preferably has a size only slightly greater than the protruding outer surface of the roller, hindering back reaming debris from entering inside the shell. In its most preferred form, both the shell and rollers tapers forwardly and have a frustoconical shape, but the forwardly inclined curved outer surface of the rollers can be achieved in several ways, such as by using cylindrical rollers on angled axles or by using tapered rollers on axles that are parallel to the axis of rotation of the frame, as further described below.
The invention also provides a method for bursting an existing pipeline using the foregoing system that can be applied even to ductile pipes such as iron and steel without the need to score or slit the pipe in advance. Such a method includes the steps of inserting a drill string through an existing pipeline, connecting the front connecting portion of the pipe burster to a leading end of the drill string extending through the existing pipeline, positioning the drill string so that the curved outer surfaces of the rollers engage an inner wall of the existing pipeline, and pulling and rotating the drill string to pull the pipe burster through the existing pipeline to burst the existing pipeline, wherein rotation of the frame about its axis of rotation results in rotation of the rollers when the protruding surfaces of the rollers engage and rupture the inner wall of the existing pipeline. The borehole may be further widened using a rear end portion of the pipe burster that has a greater diameter than the existing pipeline. A replacement pipe may be installed in the borehole, which pipe has a greater external diameter than the existing pipeline.
These and other aspects of the invention are described in the detailed description that follows.